It has been assumed implicitly by researchers in the field of rectennas (i.e., antenna-coupled diodes) that rectenna solar cells can provide efficient power conversion for the full solar spectrum. This would appear to be the case because rectennas operating in the microwave band can efficiently convert a broad range of frequencies. However, in the visible and near-infrared spectrums this is not the case.
Ordinarily diodes (as used in rectennas) act as mixers, producing sum and difference frequencies in response to input frequencies. This mixing effect would be expected to homogenize the input frequencies so that a single operating voltage would suffice for power conversion. This mixing effect increases with increasing input power. However, the maximum solar power that can be obtained by each rectenna element is approximately 0.3 microwatts due to coherence limitations. This does not provide sufficient power to produce significant mixing in the diode.
At the frequencies of visible and infrared light, the operation of an optical rectenna is fundamentally quantum mechanical in nature. A monochromatic (single-frequency) light source can, in principle, be converted to electrical power by an optical rectenna with 100% efficiency. However, this high conversion efficiency is available only at one operating voltage, corresponding to the photon energy (in eV) divided by the electronic charge q. That means that the entire solar spectrum cannot be efficiently converted to electrical power using optical rectennas operating at any single voltage.